Introduce ScreenLockDetectorBase and move platform logic into
ScreenLockDetectorLinux and ScreenLockDetectorMacOS. Rename mac files to
*_macos, add new base/linux source files, and update CMakeLists to
include them. Convert ScreenLockDetector into a factory/facade that
creates and forwards signals to the platform detector. Add refactoring
docs and a migration guide.
